1. Field of the Invention
The present invention relates generally to lighting applications and, more particularly, to a light-emitting device having at least one organic luminescent layer doped with at least one photoluminescent material.
2. Description of the Related Art
Light-emitting diodes have gained increasing interest as sources for lighting. These devices are classified into inorganic light-emitting diodes (xe2x80x9cLEDsxe2x80x9d) and organic light-emitting devices (xe2x80x9cOLEDsxe2x80x9d). The technology for LEDs has progressed significantly, and typical semiconductor-based LEDs now can emit a range of colors in the green-to-red wavelengths. LEDs emitting blue or violet light are rare. Commercial blue LEDs are based on gallium nitride (GaN) or indium gallium nitride (InGaN). LEDs have been coated with phosphor particles to produce a mixture of the primary colors, resulting in white light. However, the manufacture of these devices is still complex and costly.
On the contrary, OLEDs offer the promise of low drive voltage requirement and simple manufacture. The simplest OLEDs typically consist of three layers deposited on a transparent substrate: an anode layer, an active layer of electroluminescent (xe2x80x9cELxe2x80x9d) organic material, and a cathode layer. The EL organic material is either a low molecular weight organic material or a polymeric material having unsaturated bonds. Individual OLEDs typically emit broad-spectrum lights. To achieve a white light, prior-art devices incorporate closely arranged OLEDs emitting blue, green, and red light. These color lights are mixed to produce white light. An alternate scheme to produce white light is set forth in U.S. Pat. No. 5,294,870 which describes an organic EL multicolor display device comprising an organic EL source emitting blue light with green- and red-emitting fluorescent materials applied to different subpixel areas. This device emits different colors from the different subpixel areas by color shifting with the green- and red-emitting fluorescent materials. However, the manufacture of such microdevices is complex and requires sophisticated technologies.
Another example of an OLED is described in Junji Kido et al., xe2x80x9cMultilayer White Light-Emitting Organic Electroluminescent Device,xe2x80x9d 267 Science 1332-1334 (1995). This device includes three emitter layers with different carrier (or charge) transport properties, each emitting blue, green, or red light, which layers are used together to generate white light. However, the formation of successive layers requires a high degree of care so that the interfaces between the layers do not introduce unnecessary barriers to charge transport. In this device, the layers emitting the different colors typically degrade over time at different rates. Consequently, the color of light emitted from the device is likely to change over time. In addition, the uniformity of the light output over the emitting area of the device may be less than desirable because of imperfections at the interfaces between the layers.
Therefore, it is desirable to provide a light source based on organic EL materials that emits light at controllable wavelengths and is simple to manufacture. It is also desirable to use such light sources to produce white light.
A light-emitting device of the present invention comprises an anode, a cathode, and at least one organic EL material that is in contact with the anode and the cathode. The anode and the cathode are electrically isolated from one another. The organic EL material emits electromagnetic (xe2x80x9cEMxe2x80x9d) radiation having a first spectrum in response to an electrical voltage or an electrical field applied through the anode and the cathode. The electrical voltage or electrical field is applied by connecting the anode to a first electrical potential and connecting the cathode to a second electrical potential that is lower than the first. The organic EL material includes at least one photoluminescent (xe2x80x9cPLxe2x80x9d) material dispersed therein that absorbs a portion of the EM radiation emitted by the organic EL material and emits EM radiation having a second spectrum. When the EM radiation has the wavelength range in the visible spectrum (i.e, from about 380 nm to about 770 nm) the terms xe2x80x9clightxe2x80x9d and xe2x80x9cEM radiationxe2x80x9d are used interchangeably hereinafter. The organic EL material is formed into a thin film that is disposed between an anode and a cathode of the light-emitting device. The preferred PL materials comprise inorganic phosphors in the form of nanoparticles. The term xe2x80x9cnanoparticlesxe2x80x9d as used in the present disclosure means particles having the largest dimension less than about 100 nm. The preferred largest dimension for the nanoparticles is less than about 50 nm. The inorganic phosphor nanoparticles are dispersed in the organic EL material prior to film formation. The PL material absorbs EM radiation having a shorter wavelength and emits EM radiation having a longer wavelength. In this case, the PL material is said to perform a down-conversion of the EM radiation. Other PL materials may be chosen to absorb EM radiation having a longer wavelength and emit EM radiation having a shorter wavelength. In this case, the PL material is said to perform an up-conversion of the EM radiation. Thus, the choice of the organic EL material and the PL material provides a control over the color of light emitted by the OLED.